Means and method for electron acceleration



A ril 9, 1946. D. H. SLOAN 2,393,162

MEANS AND METHOD FOR ELECTRON ACCELERATION Filed D80. 16, 1941 6 She ets-Sheet 1 15. a 9 54 MMMH ma fl m m a H. A

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AMPLIFIER April 9, 1946. D. H. $1.0m 2,398,162

MEANS AND METHOD FOR ELECTRON ACCELERATION Filed Dec. 16, 1941 6 Sheets-Sheet 2 INVENTOR.

04 V10 h. 8404M April 9, 1946- D. H. SLOAN 2,398,162

MEANS AND METHOD FOR ELECTRON ACCELERATION Filed Dec. 16, 1941 6 Sheets-Sheet 3 5g 7 Fig.7.

'INVENTOR. 04 we H. SL014 N A 77'OENE Y3.

Ap 9, 1946. D. H. SLOAN MEANS AND METHOD FOR ELECTRON ACCELERATION Filed Dec. 16, 194]. 6 Sheets-Sheet 4 x E W X i J. 5 M M o W T I M n w 0. V

9, 394%. D. H. SLOAN 2,398,162

I MEANS AND METHOD FOR ELECTRON ACCELERATION Filed Dec. 16, 1941. 6 Sheets-Sheet 5 INVENTOR. 044 1/10 h. 81. can! D. H. sLdAN 2,398,162

MEANS AND METHOD FOR ELECTRON ACCELERATION Filed Dec; 16, 1941 6 Sheets-Sheet; 6

INVEN TOR. 04 Wu H. 5404A! BY 3 I f f r ATTORNEYS.

Patented Apr. 9, 1946 MEANS AID METHOD F03 ELECTRON ACCELERATION David H. Sloan, Berkeley, cans, assignor to Research Corporation, New York, N. Y., 'a' cornotation of New York Application December 16, 1941, Serial No. 423,173

13 I Claims.

My invention relates to amethod and apparatus for electron acceleration at high. energies and more particularly to an electron accelerator utilizing the energy of standing wave fields to accelerate electrons in quantity.

This application is a continuation-in-part of my copending application Serial'No. 364,284 filed Nov. *1, 1940, and Serial No. 418,669 filed Nov. 12, 1941, respectively. I

'Among the objects of my invention are: 'To

provide an electron accelerator of simple structure;v to provide an electron accelerator capable of. producing an effective electron beam accelerated to a velocity at which the energy is of the orderof many'million electron volts; to provide a sectional electron acceleraton'Which when enerized; will produce electron acceleration in accordance with the amount of power supplied and .the number of sections; to provide a means and the voltage maxima, meanwhile maintaining a fixed distance between nodes. For axial standing waves in an elongated cylindrical waveguide. I have found that both inductance and capacity can be very simply added by bulging the pipe at current maxima, and constricting it at voltage maxima, thereby forming a sequence of cells. I thus obtain a wave guide whose internodal distance is now such that an electron traveling at a speed close to the speed of light can traverse it in one-half cycle, of a lower frequency.

An electron traveling along the axis of such aloaded wave guide, with a velocity close to that of light can receive an acceleration in one cell, and arrive in the next cell one-half cycle later, when the fields have all been reversed. These new fields entered by the electrons will now be .in the proper direction to accelerate the electrons in the same direction as in the cell traversed onemethod of producing standing wave fields prophalf cycle earlier. The electrons can be made to erly phased to create electron acceleration therethrough; to provide a means and methodof exciting .a standing wave electron accelerator; to

provide a means and method oi driving a high-Q load; to provide a simple, efllcient and readily constructed electron accelerator, whereby electronscan be eflectively accelerated to ten or more {million electron volts for use in medical therapy, either by direct application of the produced electrombeams, or by the production of X-rays, or t or similar'purpo'ses', and to provide a method and. Dparatus foriaccelerating electrons at highen- 'sles;

Resonatorshave heretofore been used for acelerating electrons to high energy in the devel- A study of the standing wave field distribution in having. a very high impedance from node opment of powerful high-frequency oscillators.

light or an electron can travel in one-half cycle,

and-consequently such wave guides are impractical for use for electron acceleration.

, ,I have found however, that the distance bet- 5o tween nodesin an elong'ated wave guide. can be decreased for a fixed i'requency'by loading the wave guide.

Loading can be accomplished by adding inductance (volume) 'nearthe current serially traverse .a sufilcient number of cells to give the electrons a total acceleration which provides the desired energy.

I have embodied the above concept into practical devices for providing electrons accelerated to energies as high as ten to fifty million electron volts, dependent upon size and power characteristics, and several such devices will herein be described in detail.

In the drawings:

' Fig.1 is a diagram showing a normal cylindrical wave guide and the electric held 'of the standing wave pattern therein.

Fig. 2 is a diagram showing wave guide and the electric field of the standing wave pattern, loaded by forming a series of bulges and constrictions axially along the wave guide to lower the frequency.

Figures 3 and 4 are diagrams showing equiva. lent waves on a straight wire, as related to Figures 1 and 2, respectively.

Fig. 5 is a diagram showing one preferred modification of wave guide that can. be utilized for the production of high velocity electrons.

Fig. 6 is a view partly in section and partly diagrammatic showing a wave guide similar to that of Figure 5, constructed for the production of X-rays.

Fig. 7 is a longitudinal sectional view of a tetrode tube adapted to energize the accelerator shown in Fig. 5 taken as indicated by the section line ll-l in Figure 9.

Figs; a, 9 and 10 are cross-sectional views talsen as indicatedby section lines a e, and Hi -l ll maaima'orby adding capacity (surface) near respectively-in Fig. .7.

' radial waves.

where the electric field is greatest. room for more magnetic field lines, and shorten Fig. 11 is a longitudinal sectional view are triode tube adapted to energize the accelerator shown in Fig. 5.

Fig. 12 is a cross sectional view of the tubes 7 shown in Figure 11 taken as indicated by the line iii-i2 in that figure.

Fig. 13 is a longitudinal sectional view of a direct coupled triode tube and single cellhigh voltage resonator utilized for ,X-d'ay production.

Referring directly to the drawings for a more detailed discussion and description of our invention, and first referring to a conventional elongated cylindrical wave guide 9, shown in'Fig. 1,

excited by a power source at the proper freconductor and inner conductor 26 attached similarly to cell It, with loop 21 entering this adjoining cell.

Power at theresonant frequency-can now be suppliedto .the waveguide from amplifier IS with the result that a high velocity electron axially entering the' wave guide can travel from node to node in a half-cycle.

' Axially directed electrons will-{be accelerated during such passage, if properly entered, and

then the field will reverse as the electrons-enter r the next cell. The cells will be properly phased to; continueto accelerate the electron during its passage through the next internodal distance. This acceleration can be theoretically continued each acceleration the electrons become heavier. as will be pointed out later, and forces which would otherwise tend to over-focus the electron are consequently less able to deflect them beyond the axis.

electric fields at right angles to the axis and contribute nothing to the useful axial electric field intensity. Consequently each cell also requires the interference of two oppositely directed These latter waves can and do provide the axial electric field. When the resonator is energized at a frequency which will produce the indicated pattern, all fourof these waves are present. Furthermore the frequency at which all four oi these waves can coexist in the proper relative strengths, is the only frequency for which this pattern can occur.

However. in a wave guide of the form shown in Fig. l the frequency will be too high for an electron to travel between nodes in one-half cycle, 1. e., the distance from node to node for afixed frequency is greater than the distance that either light or an electron can travel in a half-cycle. Figure 3 shows diagrammatically the equivalent wave on a straight wire.

In order to gain more time, I lower the frequency meanwhile preserving the same inter nodal distance by adding-inductance where the magnetic held is greatestandby adding capacity I thus make the electric field lines.

This condition can he produced for example, by forming a series of regular bulges 3' around the wave guide and a series of constrictions 4 .amplifier' output transmission line It entering end cell it from an amplifier ii, the amplifier being preferably excited from an amplifier input transmission line i? entering an adjoining cell ill.

Amplifier output transmission line comprises an outer conductor 2t attached to wave guide 6 and an inner conductor 21 entering the wave guide as a loop 22 and returning to contact outer conduc- From a practical point of view, the extent to which this means and method of accelerating electrons can be carried'out, is dependent only upon the physical length to which the accelerator-wave guide can be extended, and the amount of power available for driving it. I will therefore next consider in detail, an electron accelerator designed to operate on pproximately 100 kw. peak power at 30 cm. wavelength and with 30 half-wave cells to'produce a dense beam of electrons having energies of the order of tenmillion electron volts for X-ray production.

Considering Fig. '6, the waveguide l is' divlded into cells as described in Fig. 5 with diaphragms I I spaced approximately 15 cm. and of a diameter preferably in the neighborhood or .8 wavelength,

7 dependent on the size of the diaphragm openings'. At the one end of the waveguide a central electron inlet aperture 30 is provided. Aperture 30 is outwardly surrounded by an anode cylinder 3! having an axial anode aperture 32 pierced therethrough. Anode cylinder BI is surrounded by an envelope cylinder 34 to which is joined,

for example, by a. metal-to-glass seal 35, a glass envelope portion to carryinga reentrant stem 31. Reentrant stem 31 has filament leads 39 passing therethroiigh supporting source 40 of alarge number of electrons alined with apertures 32 and 30, and coaxially P sitioned with respect to the axis oithewave guide i. The wave guide I is grounded, and the electron source-l0 is held.

.therebetween as shown in Fig. 2, these bulges at a negative potentialwith relation to the wave guide and anode cylinder 34 .bymeans of an accelerating source oi -Dwc. potential ll, laterto be discussed.

Axlally mounted in the other end of the'wave guide l is an X-ray target A; cooled by pipes 45, the X-rays being emitted axially from the target outside of the wave guide. Each cell is also provided Withpreferablyseparate water coolingf'pipe systems 46, in. order that thetemperature otjfeachcell can be separately controlled, as will bole?!- plained later. In this case the'entire wave. guide is evacuated. 1

While there are many ways of energizing a.

high-Q load such as that provided by the resonator-wave guide Just above described, I prefer-to utilize either a tetrode or grounded grid triode. Driving a high-Q load. such as the waveguide above described, from an oscillator of fixed frequency is impractical, because of the slight frequency drift generally encountered due to heat tor 2o. Amplifier input line I! comprises outer changing the dimensions of the wave guide. A

indefinitely as the stream of electrons will be focussed along the axis of the wave guide. With lower. frequencies the anode, the load and the grid may interact through intervening coupling circuits, and elaborate precautions are then needed to prevent the grid from being more favorably excited by energy stored in one of the line-choke support is employed prevent power escape.

In the construction here shown a. side tube 60 of relatively large diameter is welded at substanintervening coupling circuits, than by the load energy, and thus produce parasitic oscillation. However, by completely shielding the input from the output circuit of the tube, it then becomes possible to feed power to the grid directly from the loadand the grid canirecelve energyfrom no other circuits. Neutralization circuits in them; selves constitute possible parasitic circuits. Consequently, the shielding action of a grounded grid is preferred for isolating the input from the output, so that there can be only a single path from the output through the. load back to the input of the excited tube. I

As stated above, the tube usedto excite the load can be either'a triode or tetrode, preferably having in both cases, a grounded grid. I have'found that for the shorter wave lengths a tetrode is more satisfactory, because the tetrode reduces the direct feedback and prevents the input from bein excited by anode voltage arising from energy stored in the anode circuit, or in associated coupling circuits. Consequently, for wavelengths on the order of 30 centimeters a tetrode is to be 'preferred, whereas at higher wavelengths, suchas 1 meter for example, a triodeis highly satisfactory.

There'are some advantages to be gained by constructing and operating the wave guide, for the longer wavelengths, inasmuch as more volts per cell at. the same power can be obtained. This situation will be'discussed in conjunction with tially the midpoint of the accelerator grid member 5|. The side tube carries a metallic flange 6 I, with a glass insulator tube 62 fitted against it,

'andin turn carrying a terminal flange 5d.

Through this terminal flange passes a pipe 65 which project through a support aperture 56 in the side of'the accelerator grid member, and on the end of which the anode body E52 is attached. The action here will be described following the mechanical description of the anode, as the exing device.

the device as supplied by atriode and operating on the longer wavelength. Consequently, a I

.intend to utilize either triode or tetrode tubes to energize my-accelerators, I will firstconsider the case where a tetrode is utilized to energize the wave-guide at a wavelength of 30 centimeters in accordance with the description previously'given above. Such a tube is shown in Fig. 7, this figure being. a combination of Figures 18 and 8 of my copending application Serial No. 364,284 filed November 4, 1940, and entitled "High frequency electronic tube, cited above.

Referring to Fig. 7 and the associated section views of Figs; 8, 9, and 10 ananode cavity resonator is formed by an accelerator grid member 5|, surrounding an anode body 52.

The support of this anode body is from the midor quarter wavelength point of the anode body, i. e., at a potential anode, so that there is little tendencyfor power to escape from the support structure. Such tendency as there is for power to leak from the support point is sup-' pressed by either-(or both of two methods. .First, and preferable is the casewhere the tube may 7 be predesigned to operate at a fixed wavelength,

is a movable plate 53 mounted on the sliding rod 54 passing througha Wilson seal 55 in acceler Owing to the desirability for providing cooling, the body itself must be water-tight, and accordingly it is constructed of a flared cylinder ill, to the flared end of which the anode face H is hardsoldered. The other end' of the cylinder is closed by a threaded disc 12. e

' Thesupportlng pipe enters the flared cylinder 18 through an aperture in the side thereof. The end of the pipe i threaded into a boss 16 on an inner baiile cylinder which boss is soldered to the inner wall of the cylinder 70. The boss M extends internally to form a cylindrical chamber ill, which connects by a side pipe ll through the end 78 of the baiile cylinder 15, so that water introduced through the pipe H is discharged directly against the active face ll of the anode, and thence is forced around the exterlor of the bafile cylinder to reenter its open end.

It can then return within the cylinder to enter the open end of a return pipe 10, which is mounted concentrically within the pipe 55 by means of pansion between the two. Water is introduced half wavelength line, but additional space is needed for the insulating cylinder F52, which must ator grid member 5!. and making contact with the accelerator grid'member Elby means of a spring skirt 51- Plate 53 maybe adjusted to bring the node ofthe resonant line accurately at the point of anode'body support. The plate 58 may however be more useful as a tuning device,

andtherefore the principle of a transmissionwithstand the full D. C. anode potential of 20,000 .volts or more. The length of this section is measured from the anode and its housing, and the impedance at its outer end is very high. so that looking into it from the anode the impedance is also very high.

This high impedance is connected in shunt across the line formed by the anode body 52' and accelerator gridmember 05 very near the nodal point, where the impedance of the latter line is low, and accordingly a very small portion of the current flowing at this point will take the high impedance path to the outer world.

In other terms, the full wave line is connected so nearthe node of the main anode oscillator circuit that only a few volts are effective across its termini, and therefore very small currents will tend to flow therein,.representing a power loss the tube.

conductor 2 l.

of W/Z where V is the small input voltage and Z the large input impedance. Moreover, since the line is one wavelengthlong, only the small voltage V will be effective to'cause radiation from the radiating system constituted line.

. It has already been stated bythe end of the that fine .tunlng of the anode resonator can be accomplished by of increase in capacitance, and hence the over-' all effect is to raise the frequency of operation of The extent of the efiect produced by the shell may be increased by chamfering the end,,wherethe effect on' the capacity isv greater than on the inductance. Because of' possible differential expansion the shell is solidly secured.

to the anode body only at the anode end, contact with the opposite end flange being made frictionally by resilient fingers in formed by slotting the end of the cylinder 90 longitudinally.

. Radio frequency power developed in the anode circuit may be transferred to wave guide I by the outer conductor of the amplifier output linev It, welded or otherwise sealed to the side of the accelerator grid member with central conductor 2i of line it entering the side of the accelerator grid member through an aperture 92 and bent as a loop t l, parallel to the axis of the anode body substantially at thepotential node (and current loop) of the anode resonator 50.

It may be convenient to make the loop '90 of high impedance antenna, fed by a low impedance smallbore metal tubing so that it may be water cooled by liquid carried in through the central In this case the tubing may be brought out of the anode resonator as shown, and coiled around and soldered to the outer conductor 20 of the line it to cool it also between the tube and the wave guide 9.

The envelpe comprises an accelerator grid supporting flange 500, which is built with a skirt IEJI. This isfollowed by an insulating cylindrical housing section 902, a control grid flange I06, another short insulating section I05, and a filament support flange I06 in the order named. The latter flange l06is dished downwardly, and through it passes the main central filament support column, which comprises. an outer tube I01 and inner tube Hit, the ends of which are set in a counterbore and soldered or welded to an end flange H0. Against this flangeabuts an insulating gasket I I i, and the end -plate H2 of the tube is held between this gasket and a second gasket H t by means of a clamping ring l I5 and tie-bolts HE, thus completing the outer envelope of the tube. J

The central supporting column I01|00 holds the outer ends of a pluralityof radially extending laments IZt, which are set in a terminal ring umn are fiat and are indicated by the reference character I2I. in Fig. 8. The space formed bei231 respectively, these pipes entering at the plane of a ring 128 which also carries a connecting lug I for the filament lead. The two channels .I2I are connected at the top of the colun'mJ The inner ends of the filaments I20 are sup-. ported upon L-shaped lugs I carried on the ends of support pipes I3I. These pipes extend,

I36 which is three-quarters wavelength long and '-terminates closely adjacent the filaments.

Its lower end is closed by a conducting disc I35, supported by and spaced from the bottom flange I I2 by both a tie-bolt I36 and a spacer pipe I31, with which the disc I and the flange II2 are electrically continuous, so that they form the other end of the filament circuit, the flange II2 being provided with an axial lug 339 for connecting a filament lead. The end flange II2 also carries a conducting cylinder 0, which is a quarter waveliengthaslong and terminates slightly below the rec The water pipes and connecting lugs form a quarter wave line comprising the conductors I08 and I00 which give it negligible series impedance and so form a by-pass which makes the tube I40 effectively continuous with the tube I08 at high frequency although insulated against filament supply frequency and. voltage. The tube I40, together with the water pipes I3I and spacer tube I31 form a closed end quarter wavclength line, presenting a high impedance when viewed from the filament end. a

A similar series-line by-pass effect to that offered by the lower quarter wavelength-section .is offered by the upper three-quarter wavelength section formed by the conductors I08 and I30, and in like manner also the filament-support pipes I3I form the inner conductor, and the three-quarter wavelength cylinder I30 forms the outer conductor of the three-quarter wavelength line, closed at the end, and ofiering. veryhigh impedance. R. F. transmission from the internal or fllament support structure is: therefore prevented.

The grid I is supported-at the top of a cylindrical column I5I, borne hyinner extension I52 l of the grid flange I04. The grid takes the form' .120. Two sides or the inner tube loaor the colof a flat plate in which the grid-apertures are slots I54 whose edges form the surfaces giving the propercurvature of the field. 'A quarter wavelength series-line .by-pass and choke is formed by atub'ular skirt I55 mounted on the.

filament support column. tube I01. The gridfilament circuit is brought to exact resonance; the closed end, three-quarter wave transmission line comprising the skirt I55, grid column I5I, the

grid I 50 and filaments I29 themselves, being tuned to exactly three-quarter wavelength, This tuning-is done by sliding the skirt I55 on the filament support tube column I01 to get the final exact adjustment which gives the necessary high impedance, at the grid. The adjustment is made by means .of a concentric water fitting comprising an external pipe I55 and an internal pipe I51 which circles and colls the skirt I55. The outer pipe emerges through a Wilson seal I50 in flange I06, so that the skirt can be adjusted, using the external extension of the fitting as a handle.

Tuning witha slider of this type, which in itself is practically a quarter wave choke, has the great advantage that the junction between the slider and its supporting column necessarily comes substantially at a current node. Accordingly there is much less difiiculty with heating at the sliding contact than where such contact occurs in the vicinity of a current loop.

The grid W is operated at radio-frequency ground, and may itself be considered as a boundary grid terminating the grid-filament line, while it is the filament which swings in potential with relation thereto. The accelerator grid I 60 comprises a relatively thick plate with apertures lBI for the passage of the electrons. The quar- This tube is designed for operation with the grid I60 at ground potential, and with the filament at accelerating potentials negative to ter-wave open-end by-pass line section between A control grid use and accelerator grid IE0 is terminated at the interspace between the two, and comprises the space between the cylinder IEI supporting the grid Ilia and a short length of tubing I82 I which is mounted on annular spacers W5 and its on a cylinder Itt fixed to the interior surface ofthe flange Mil. Another annular spacer ltl within cylinder Ito carries an upwardly extending skirt ltd which terminates a small distance below the by-passing line I 62. Another short cylindrical section of conductor llil is mounted concentrically with the grid col- -umn within, above and below, the flange I lit.

This conductor acts as a part of a choke, but its primary function is to displace the point of radiofrequency stress from the seal.

Again proceeding from the lower end of the device upwardly the section comprised of the conductors I55 and till is a closed-end choke, preferably quarter wave, offering high input impedance to the section above it. Feeding into this high impedance section is an open-end, quarterwave blocking or by-pass section comprising conductors 55d and its and making these conductors electrically continuous, so that conductors I66 and tell become the high impedance closed-end quarter wavelength line into which the control grid-accelerator grid section feeds. This impedance gives the grid-accelerator section, as an open-end, one-quarter wavelength line, negligible impedance, and effectively shorts the two grids at radio-frequency, although they are insulated against their ill-E1. potential differences.

The grid-filament circuit is driven by means of a coupling loop Elie formed on the end of conductor it brought in throu h exterior conductor of amplifier input line ll through the flange lfil Where theloop emerges from the line Ill its grounded side is brought in coplanar with the control grid disc 65%, lyin in a slot MI in the disc as shown in Fig. 9. The loop itt is located not far from the potential node and current loop in the grids-filament line, and its size is proportioned to give a satisfactory input impedance. By

means of this loop the cathode resonator is driven from the transmission line ll connected to the wave guide i, so that the tube as a whole acts as an amplifier, supplying power to the wave uide and driven therefrom.

Cooling of the accelerator grid is accomplished by a water pipe I85, brought in through the'flange I'll and forming a single turn in contact with the grid disc ltd. -Another cooling pipe I86 is,

brought in through the flange HM, encircles the central column slightly below the filament plane, in contact with the outer conductor I5I of the grid-filament line.

' is connected to filament connection lugs I25 and ground. The grid I50 operates at R. F. ground potential and may be considered as 9, boundary grid terminating the grid-filament line, while it is the filaments which swing in potential with relation thereto. Owing to the method of coupling the grid circuit, it is extremely sharply tuned. The tube is accordingly critical in adjustment, but the number of adustments is low, namely, the tuning of the anode circuit and the tuning of the sliding skirt I55, and the value of the accelerating voltage.

The tube is energized by connecting the anode body to one end of a pair of series connected transformer secondaries I90, the other end being connected to filament connecting lug E25, the midpoint I9I of the secondaries being grounded. Flange Itt is grounded, placing grid Hill at ground potentiall The filaments are supplied by filament transformer I92, the secondary of which I39. At the higher frequencies, within the range of operation, high anode voltages, to give high powers, will give half-cycle transits along the lengths of path here provided and I prefer to use this mode of operation of the tube to energize wave guide I at 30 .cm. wavelength, the anode voltage being adjusted to give the proper transit time. Grid I50 is connected through grid leak I at to ground.

The tube therefore acts as an amplifier-oscillator, delivering power directly to the wave guide through loops M (Fig. 7) and 22, (Fig. 6) and being excited directly from the wave guide by loops 2! (Fig. 6) and I66 (Fig. '7) through transmission lines Hi and I1, respectively. There is no other path for energy between the oscillator and the load and'therefore no parasitic oscillations de velop. I prefer that the wave guide load and excitation loops 22 and 2? be coupled to the wave guide I in adjacent cells, so that the phase will reverse and prevent oscillation at the nearby frequency for which the standing waves are not oppositely polarized.

In operation of the accelerator, the tetrode and circuit as described is energized to excite the wave guide I. Filament id is heated, and anode source it is connected. Electrons from filament till thus enter the wave guide and start along the axis thereof. The electrons may be grouped, if desired, into synchronized pulses before entering the accelerator system by modulating the electron un with R. F. from the anode transmission line through a conventional grid not shown. However, in most cases such a grouping is not necessary.

After the electrons enter the first section I2 of the wave guide, they are accelerated along the wave guide until they reach X-ray target M, receiving an additional acceleration as they pass through each cell of the wave guide. With three kilowatts peak power per cell, each cell will de-- velop, at 30 centimeter wavelength, about 500 kilovolts, about .7 of which can act usefully upon an electron passing through the cell. Thus 30 cells with kw. peak. power delivered by the exciting oscillator, will give electrons in the beam impacting the X-ray target an acceleration on the order of 10 million electron volts. As each interior cell is approximately 15 cm. long such an accelerator has an overall length of only 450 cm. or roughly 15 feet.

The design of the initial cells of the wave guide will depend upon the velocity'with which the electrons enter the wave guide. An electrostatic generator can be used for example as a source lower velocities, then thefirst few cells in the wave guide may have shorter lengths in order to bring the entering electrons up to a. velocity that is close to the speed of light and nearly constant, before entering identical cells.

It is also important that the diameters of all identical cells be kept nearly exactly equal as possible in order for each of them to have equally strong radial reflections at the frequency 'of the combined coupled system. The electric field of a cell having excessive energy therein will spread into the adjoining cells, creating a greater effective distance between nodes and consequent deficient capacity. This can be corrected by increased inductance obtained by increasing the diameter of the cell. Ihave found in most cases If however, the a 30 cm. excitation, two exciting tetrodes as described should be utilized, each ieeding into and excited by adjacent cells, respectively, the second tube feeding cells placed intermediate the ends of the wave guide.

, It is not important where the wave guide is energized nor. is it important how many tubes are utilized to feed it. However, in order to avoid complexity and duplication of parts, it may be preferable to utilize a longer wavelength and thus obtain more volts per cell, and to utilize a that the diameters of the cells are automatically maintained exact, as those which are smaller receive too much energy, and overheat. Consequently they expand and enlarge to proper size. In accordance with this action, it discrepancies are too great for automatic control, very accu-' rate control of the'cells can be obtained by. controlling the amount of cooling water delivered to each cell of the line, and separate cooling systems are provided tor-this purpose.

Thus it will be seen that I have provided an electron accelerator in, which the entering electrons are accelerated axially to a velocity appreaching that of light. As the velocity of the accelerated electrons approaches a limiting value equal to the velocity of light, the velocity increases very slowly with increasing energy. Further acceleration results therefore, chiefly in in creasing the relativistic mass of the electrons, such high velocity electrons produce very penetrating X-rays, and these high velocity electrons are produced in large quantities. The efiective increase in mass greatly aids the focussing action of the wave guide, as the electrons are less able to be deflected from their-axial courses as they pass along the accelerator, A large percentage consequently impact the target. I

-While I have described the above device as designed for generating copious numbers of electrons having accelerations on the order of 10 M. E. V. for the production of X-rays, it is in many cases highly desirable to produce efiective electron accelerations of 15 to 20 M. E. V. in order that an electron beam can be directly at plied to living tissues. Electrons at 15 to 20 M. E. V. deliver less energy per millimeter of path than do electrons at lower accelerations.

Consequently such bean-is can be applied directly toliving tissues for release of their energy below the surface of the. tissue without seriously dama dng the surface.

By utilizing approx'mately double the number of cells in the device sunilar to that previously described, electron energies of 15 to 20 M. E. V. can be produced. This would only iiivolve the use of an accelerator aboutfeet long. This accelerator would however, require double the power; If electrons at 15 to 20 M. E. V. are to be produced in such an accelerator with single triode for the excitation of the wave guide.

At 1 meter for example, each cell will be approximately one-half meter long. Fifty such cells, make the wave guide approximately 25 meters long and approximately .8 meter in diameter, which structure is not excessively large when it is considered that 15 to 20 M. E. V. electrons are produced thereby. The general construction of such a wave guideis exactly similar to that which has, been previously described. The only difference, except for dimensions, will be the type of tube I prefer to utilize to excite the wave guide at the lower frequency. A triode suitable for this purpose has been described in my .copending application serial No. 418,669 filed Nov. 12, 1940, entitled.I-Iigh frequency triode oscillator, and is shown as modified for use as an amplifier/in Fig. 11 herein, operated at a peak power giving 15 to 20 M. E. V. electrons in the 50 cell accelerator, on the order of kw.

Referring to Figs. 11 and 12 for a more detailed description of a triode suitable for energimng wave guide I, concentric inner and outer' fila-. ment supports 20!! and 20! are provided and these supports may be, if desired, water cooled.

The axially disposed filaments 2% are supported on one end thereof by blocks 2% attached to the inner fllament support flangeddii by a plurality of spaced layers oi. resilient fins 2% so that the filaments can expand and contract during heating without exerting pressure between filament supports are and 2M.

Due to the resilient filament connection, inner and outer filament supports can be sealed together by theme of spaced annular insulating rings row and 2%, such as of glass or ceramic,

separated by a rubber ring 2%. These rings are slidably positioned around the inner filament support 2% and are spaced from the fixed base of an outer cathode choke Zillby a short spacing sleeve 2i i. Along spacing sleeve did is then led outwardly along support 2% and slidable thereon 'to the inner filament support 2%. The pressure applied by screws are forces theglass elements of the two sets of glass rings together, thus ex-, panding the intermediate rubber rings against filament support 2% and a-seailng sleeve 226 in wardly attached to outer filament support Edi. An efiective and vacuum tight seal between the inner and outer filament supports res and see is thus procured.- The inner and outer cathode supports are flrmlytied together and, due to theresilient support of filaments 2M, filament breakage during tube transport is eliminated.

The grid 225 is shaped and positioned to sur- I coupling loop l8il.

round the filament support and the filaments 202 register with grid slots 226 for electron passage to the anode. Grid 225 is supported by a heavy frame 227, this frame being connected to filament support flange 229 by a glass cylinder 230, thus completing the tieup between the grid and the filament supports. A choke-line-choke system 23l is provided in the space between the outer filament supports and the grid 225 to prevent loss of radio-frequency and the space between filament support flange 205 and the outer filament support 2M is shorted to R. F. by quarterwave cylinder 232. grid is closed by a grid cap 235. resonator 233 is thus formed.

Attached to the outside ofthe grid 225 above the lateral flange 22l, is a second lateral grid flange 236 extendedoutwardly in steps to support a cylindrical glass anode insulator 231, the top of which is closed by an anode disc 2%, through which pass a plurality of internally looped water cooling pipes M0, the loops 2M being welded to anode cylinder M2 closed by anode dome M3. Adjacent pipe loops 2M are connected on one side by anode material and each such connected pair of loops are spaced so that the spaces come opposite the grid slots 22b and the filaments 2%. Thus, the electrons emitted from the filaments. after being decelerated, pass between adjacent loops of the water cooling pipes into the space 2% between the anode it? and the glass cylinder 23?. The glass charges up and repels the electrons, which are then collected by the outer surface of the anode and the pipes themselves. Inasmuch as the electrons become widely dispersed within this space, no hot-spots can appear on the anode atv any point.

Anode potential is supplied to the anode disc 2% through connection 2%. Inasmuch as it is desired to operate the grid of this tube at ground potential, a permanent connection of a vacuum line Z lli can be made to the grid structure between flanges 22W and 236.

I prefer to terminate the anode resonator 250 in an inner quarterwave choke and line section A A cathode-grid followed by an annular quarterwav line B; a

choke C followed by a quarterwave line D: backed up by a quarterwave choke E, line F and final choke G, these lines and chokes being annularly disposed.

Such a termination of' the anode resonator permits the power take-off loop 96 to be inserted through frame 22? and the stepped flange 236 of the grid at a region of maximum current in the first choke A, which is also a part of the anode resonator. The cathode resonator is closed to R. F. by quarterwave anti-resonant chokes X and Y spaced on filament support till by an electrical quarterwave line portion thereof.

The input transmission line ill from the wave guide I passes through frame 221? close to output line it, and exterior conductor 25 of input line H i attached to grid 225 around an a erture 25i through which internal conductor 26 may pass and return to external conductor 25 to form the The anode resonator th refore directly feeds the load, and the cathode resonator is excited directly from the load.

The inner end of the time approaching zero half cycles, due to the lower frequency at which the tube operates.

In this case X-ray target 44 on the end of wave guide I may be dispensed with, substituting at the same position, any conventional electron permeable window so that the electron beam may pass out of the wave guide into the surrounding atmosphere.

The grid is grounded, thus providing complete shielding, (except for the anode-cathode capacity, negligible under these conditions) between input and output circuits. The tube operates as a tuned-anode, tuned-cathode amplifier directly feeding into and directly excited from the waveguide, and no parasitic oscillations can be pro duced.

I have described above, a type of electron accelerator having multiple cells. The same general principles of operation however can be applied for electron acceleration in a single cell when lower electron energies are satisfactory.

In Fig. 13 I have shown such a device based on the triode just above described, where the oscillator is directly connected to the load, which in this case is a single cell half wave resonator 2% formed as a closed continuation of cylindrical anode 242 of a grounded anode tube somewhat similar in construction to that shown in Fig. 11 The tube proper comprises filaments 29?. mounted on filament supports 2% and 2M, and surrounded by a grid 225 having slots 226 therein through which the emission from filaments 2&2 reaches the anode 2M. The filament structure and the inner grid surface form the cathode resonator Anode 242 is energized by connection of one end of secondary 252 of an anode transformer 253 to anode connection Mt with the-other end of secondary 252 connected to outer filament supl port 20i through a resistance 2%. Frame 22! supporting grid 225 is grounded and also connected to resistance 2% intermediate the ends 233 and the outer grid surface and the anode form the anode resonator 253. Both of these resonators are closed to R. F. at the outer ends by high impedance anti-resonant choke systems 286 and 285 respectively as used to close the grid-filament resonator 233, in Fig. 11.

Half -wave resonator cell 289 is connected to anode 242 by a constricted portion 293 and the adjacent end of the grid 225 is preferably provided with a divided end portion 29L both portions of these end walls being provided with registering axial apertures 29%. The division of the end of the grid permits both anode'and cathode l'esonators to be shaped and tuned so that both will have a standing wave voltage loop registering over the filaments and the grid slots, preferably an electrical one-half wave length from the axis of the curved grid and anode ends.

The anode resonator is shaped by constriction 29!] to join the single cell resonator 230 at junction points where the currents in the adjoinin resonators are equal when ,the anode resonator voltages are in correct relationship to develop and deliver full power to the single cell high voltage resonator 280.

The tube in this case is preferably energized by connecting one end of secondary 390 of anode transformer 3M to anode 262 which is grounded. he other end of anode transformer secondary 38b is connected through a bias resistor 302 to one side of the filaments which are supplied bv filament winding 3% on filament transform r 3%. The tube when energized operates as a tunedcathode tuned-anode oscillator with the anode and cathode resonators coupled only by the a ode-cathode capacity with the grid as an untuned D. C. biased structure floating at R. F. potentials induced in it by the anode and cathode resonators. The anode voltage, for the higher frequencies is adjusted for a one-half cycle transit time as for example when exciting high voltage ergize a larger and properly dimensioned high voltage cell 280, for example at one meter Wavelength, using however in this case a transit time approaching zero half'cycles. i. e., a transit time of l0-30. When the single cell resonator .280, is energized by the direct connected tube as described, there is no other nearby frequency at which the anode resonator can resonate; and in thetub'e shown the tuned cathode resonator can be correctly and unambiguously excited through the simple anode filament capacity feedback.

In order to make proper use of the high voltage developed in the single cell 280, an electron emitter 295 is positioned on the end of the inner filament support 200, in the intense electric field .at that point, to produce a stream of electrons which will pass through end apertures 29% in the grid. This stream of electrons will be accelerated through the resonator cell 280, by the standing wave fields exciting therein as described in the prior modifications of 'my device, to produce X- rays when impinging on X-ray target 3B0 positioned in an axial extension 3 of the end wall N2 of the high voltage resonator cell 280. Due to the high power dissipated in the device I prefer toprovide anode 252 with an anode cooling system sit, the cell 2% with a cell cooling system sit, and the X-ray target em with a target cooling system Sl'l.

Inasmuch as the electrons passing thrdugh resonator cell 2% do not enter at a speed approaching that of light, the internodal distance is made to be less than one-half wavelength in order that the electrons may traverse the axis of the space in a one-half cycle.

Thus, in the example shown in Fig. 13, avery simple, compact electron accelerator can be produced. The peak power for the single cell can be made to be very high, as the entire power of the tube is utilized in the cell 28% A million-volt X-ray tube can thus be produced with a very simpie structure, with approximately 106 kw. peak power supplied thereto. However while I have shown high voltage resonator cell 280 energized by a triode, a tetrode such as that shown in Figure 7 can also be used in case greater emciency is desired. The cell 230 can be connected to the anode exactly as in the case of the triode, but in this instance a transmission line such as line ll is used to excite the grid cathode resonator directly from the cell see in order that proper in= put be supplied.

Furthermore. it will be seen that, if desired, the X-ray target did of the single cell high voltage resonator we can be removed and the electrons passing through the cell 2'80 can be injected through entrance to of the aceelerator shown in Fig. 6, replacing the electron gun there shown, to provide an initial beam acceleration on the order of l M. E. V.

If a tetrode is used as suggested just above, the

emciency and power of the tube may be sufiicient to permit the use of several cells in series similar to those of wave guide 5, provided with diaphragms having graded spacings to allow proper transit of electrons along the axis of the cells as the electrons are accelerated. Thus my invention may take the form of a single direct coupled cell, multiple direct coupled cells or multiple cells directly driven from separate amplifiers, with final to be distinctly understood that this term is used electron energies limited only by A.-C. power available, number of cells utilized, and the frequency of oscillation.

While the conventional term "transit time has been used throughout this specification, I wish it herein as meaning the period elapsing between the time of maximum emission from the filaments and the time of maximum work performed by the emitted electrons on the load field. This distinction is made, inasmuch as in some cases the actual electron transit time from conductor to conductor is not necessarily the true electron work period, as for example whenthe electrons travel part way while fields are near zero of cycle.

My invention'possesses numerous objects and features of advantage, some of which have been setiorth in the foregoing description of specific apparatus embodying and utilizing my novel method. It is however to be understood that my method is applicable to other apparatus, and that I do not limit myself in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appendedclaims.

I claim:

1. In combination with an electron discharge tube oscillator having a tuned resonator formed between an inner and an outer element of said tube, an axially symmetrical substantially-closed member having a substantially clear interior chamber axially joined to said outer member around a'communicating axial aperture, and an electron emitting surface mounted on said inner tube element, and means for energizing said tube to create standing waves in both said resonator and said chamber with the field of the standing waves in said chamber directed to accelerate electrons from said source through said aperture and axially through said chamber to impact the 'wall of said chamber opposite said aperture.

2. An electron accelerator comprising a hollow elongated substantially closed wave guide, diaphragm means dividing said wave guide into a plurality of separate axially intercommunicating cells, the end cells being of quarter wavelength with intermediate identical half wavelength cells,

means for energizing said. wave guide to produce standing wave fields therein with the nodes thereof registering with said diaphragms, and means for axially introducing high velocity electrons into one end of said wave guide.

3. Apparatus in accordance with claim 2 wherein the diameter of said wave guide is on the order of .8 "wavelength.

4. Apparatus in accordance with claim 2 wherein each of said cells is provided with a separate cooling system.

5. An electron accelerator comprising an elonated axially symmetrical substantially closed ,wave guide having an axially clear space therein,

a plurality of inwardly extending diaphragms dividing said wave guide into cells, the end cells being electrically of quarter wave length, with intermediate electrical half wavelength cells, said wave guide being loaded by said diaphragms to provide internodal distances such that an electron can traverse such distances when travelling at a velocity less than that of light, alternating current supply. means for energizing said wave guide to create a standing wavepattern therein having nodes registering with said diaphragms, one end of said wave guide having an axial aperture therein, an electron emitting surface positioned outside of said wave guide in alinement thereof entering the periphery of one of said cells.

7. Apparatus in accordance with claim wherein said alternating current supply means is an electron discharge amplifier tube having an anode, cathode and a grid, and wherein the grid of said tube is grounded with the anode thereof directly coupled to said wave guide by an output transmission line, said cathode being excited directly from said wave guide by a second transmission line.

8. Apparatus in accordance with claim 5 wherein said alternating current supply means is an electron discharge amplifier tube having an anode, cathode and a grid, and wherein the grid of said tube is grounded with the anode thereof directly coupled to said wave guide by an output transmission line-said cathode being excited directly from said wave guide by a second transmission line, said transmission linesbeing respectively coupled to said wave guide in adjacent cells.

9. Means for driving a high-Q load such as a hollow resonator comprising an electron discharge tube amplifier having a tuned anode circuit, a tuned cathode circuit, and a grounded grid shielding anode from cathode except for electron permeable portions of said grid, means for directly coupling said anode circuit with said load and means for directly exciting said cathode circuit from said load.

10. Means for driving a high-Q load such as a hollow resonator comprising a pair of coaxial transmission lines each having an outer conductor connected to the wall of said load resonator and an inner conductor entering the interior of said loadresonator, a vacuum tube electron permeable grid connected to both outer con ductors of said lines, an anode on one side of said grid, a cathode on the other side of said grid, said anode and said cathode being shaped to form the internal conductors of anode and. cathode resonators respectively, the material of said grid being'extended to form the external conductors of said latter resonators, the internal conductors of said transmission lines separately entering said anode and cathode resonators as output and exciting loops respectively.

11. Means for driving a high-Q load such as a hollow resonator comprising a pair of coaxial transmission lines each having an outer conductor connected to the wall of said load resonator 'and an inner conductor entering the interior of said load resonator, a vacuum tube electron permeable grid connected to both outer conductors of said lines, an anode on one side of .said grid, a cathode on the other side of said grid, second electron permeable grid intermediate said cathode and first grid said anode and cathode being shaped to form the internal conductors of anode and cathode resonators respectively, an extension of said joined outer transmission line conductors shaped to form the outer conductor of said anode resonator, an extension of said first grid shaped to form the outer conductor of said cathode resonator, the internal conductor of one of said transmission lines entering said anode A resonator as an output loop, the internal conduc- -tor of said other transmission line entering said cathode resonator as an exciting loop.

12. Means for driving a high-Q load such as a hollow resonator comprising a pair of coaxial transmission lines each having an outer conductor connected to the wall of said load resonator and an inner conductor entering the interior of said load resonator, an electron emitting cathode shaped to form the inner conductor of a cathode resonator, a grid shaped to form the outer conductor of said cathode resonator and having an apertured portion registering with said cathode, an anode positioned to receive electrons from said cathode through said apertures, and forming one conductor of an anode resonator, a conducting surface forming the other conductor of,

said anode resonator.

13. An electron accelerator comprising an axially symmetrical substantially closed conducting member having a substantially clear interior chamber therein of approximately one-half wave in length and approximately .8 of a wavelength in diameter, alternating current supply means for energizing said member to produce a standing wave pattern in said chamber having an axial electron accelerating field, said alternating current supply means including a driven res-.

onator axially connected to said chamber around an aperture in said member, and means positioned within said driven resonator for directing electrons through said aperture into said chamber.

DAVID E. SLOAN. 

